Variable frequency signal apparatus having a stabilizing reference signal



Dec. 20, 1966 D. HOWARD ET AL 3,293,559

VARIABLE FREQUENCY SIGNAL APPARATUS HAVING A r STABILIZING REFERENCESIGNAL Original Filed Nov. 13, 1963 I? 2 19 3 q Y r S CRYSTAL FREQUENCYMIX fo+8 HAPRMONIC U +8 STANDARD f DIVIDER HASE n K DETECTOR RF SIGNALout u (L08 VFO SOURCE vw f 2o Q a n L9 V FINE COARSE I TUNING TUNINGFREQUENCY M8 8 HARMONIC DIVIDER m mx 7 PHASE K (;)8 DETECTOR 28 25% 3 7NIB/0 DIVIDER R a GATE W GENERATOR T=K DIVIDER & GATE GENERATORREFERENCE 3 SIGNAL GATE 4 SUPPLY 35 ERROR NETWORK o INVENTORS DARWINHOWARD emcomo VARGlU AGENT United States Patent 3,293,559 VARIABLEFREQUENCY SIGNAL APPARATUS HAVING A STAiEilLIZING REFERENCE SIGNALDarwin L. Howard, Colorado Springs, (3010., and Giacomo Vargiu, RedwoodCity, Calif, assignors to Hewlett-Packard Company, Palo Alto, Calif., acorporation of California Continuation of appiication Ser. No. 323,320,Nov. 13, 1963. This application Mar. 18, 1966, Ser. No. 541,897 7Claims. (CI. 331--2) This is a continuation application of pendingapplication Serial Number 323,320 entitled Variable Frequency SignalApparatus, filed on November 13, 1963 by Darwin L. Howard and GiacomoVargiu, now abandoned.

This invention relates to signal generators and more particularly to animproved signal source which remains phase locked to a referencefrequency as the frequency of the output signal is varied over a widerange.

Certain signalling applications require the use of signal sources havinga high degree of frequency stability. This is conventionally achieved infixed frequency applications by locking the output signal of a signalsource to a frequency standard. In applications where the frequency ofthis output signal must be varied over a wide range, a high degree offrequency stability may be achieved by locking the output signal tosuccessive harmonies of a lower frequency standard.

In many of these applications it is also necessary to provide a directindication of the frequency of the output signal. Of course, highlyaccurate indications of the frequency of the output signal may beobtained by counting its frequency directly. However where the frequencyof the ouput signal is of the order of several kilomega cycles, directcounting would be extremely difficult in the present state of the art.Also, where the frequency is low and several places of precision arerequired, the gate time becomes very long. Instead of direct counting, abeat frequency is produced between a local oscillator and the highfrequency signal to be measured, which beat frequency is measured usinglow frequency counting techniques. However, schemes of this type requirethat the number of the harmonic upon which the beat frequency isestablished be identified. It becomes extremely difiiicut to identifyone harmonic number from an adjacent one where the harmonic numbers areof very high order. In these applications it is desirable to provide acounting scheme which provides a direct indication of the harmonicnumber and an indication of the variation between successive harmonicnumbers.

Accordingly, it is an object of the present invention to provide asignal source which can be phase locked to a harmonic of a referencesignal over a wide band of frequency variations.

It is another object of the present invention to provide an indicatorcircuit which provides an accurate indication of the output frequency ofthe signal source.

It is still another object of the present invention to provide improvedsignal apparatus which can be operated as a source of output signal andas a frequency meter for an applied signal.

In accordance with the illustrated embodiment of the present invention,a conventional harmonic phase detector which is responsive to thevariation in phase between an applied high frequency signal and aharmonic of a reference signal is connected in a feedback circuit tomaintain the high frequency signal in phase lock with a harmonic of thereference signal. This same reference signal is combined with a lowfrequency signal to form a resultant signal which is then compared withthe high frequency signal in a second harmonic phase detector of similarconventional design. The output of this second harmonic phase detectorcontains information about the harmonic number to which the referencesignal is phase locked and information about the low frequency signalwhich varies over the range of frequencies between successive harmonicsof the reference signal. A low frequency counter operating for a gatetime which is related to the low frequency signal counts the signal atthe output of the second phase detector and displays the harmonic numberdirectly. A second counter operating for a gate time which is related toa fixed frequency signal counts the signal at the output of the secondphase detector and displays the low frequency variation whichoccursbetween successive harmonic numbers. The combined readings,altered by suitable scale factors, thus provides a direct indication ofthe frequency of the high frequency signal. The usual :1 count ambiguityassociated with gated counters is eliminated in the harmonic numbercounter of the present invention because the gate time and countedfrequency are integrally related.

Other and incidental objects of the present invention will be apparentfrom a reading of this specification and an inspection of theaccompanying drawing which shows a block diagram of the frequencycounting system of the present invention.

Referring now to the drawing there is shown a source of high frequencysignal 9 which is adjustable over a wide range of frequencies using thecoarse tuning control 11. This signal is applied to an input of each ofharmonic phase detectors 13 and 15. These detectors may be ofconventional design similar to instantaneous signal samplers as shown inthe literature (see: gate 15 of US. Patent 3,011,129, issued on November28, 1961 to K. B. Magleby et al. and APC with Pulse Reference, T. J.Rey, Lincoln Laboratory Report 476-0019, November 9, 1960, Chapters 5and 7). The output frequency of the variable frequency oscillator 21 isdivided down by divider 23 and is mixed in mixer 19 with the output fromcrystal standard 17 to produce a resultant signal which is applied tothe other input of the phase detector 13. The output of the. phasedetector 13 is applied through switch 26 to a frequency-determiningelement of the signal source 9. In an alternate mode of operation thisoutput signal is applied to a frequency-determining element in thevariable frequency oscillator 21. Mixer 25 receives the output ofcrystal standard 17 and a frequency which is related by the divisionratio K of divider 23 to the signal at the output of the variablefrequency oscillator 211. The output of the mixer 25 is connected to theother input of phase detector 15. The output signal from the phasedetector 15 is applied through gate 27 to a first counter 29 and isapplied through gate 31 to a second counter 33. The gate 27 receives agate signal from the divider and gate generator 35, the period of whichis related to the frequency of the signal from variable frequencyoscillator 21. The gate 31 receives a gate signal from divider and gategenerator 37, the period of which is related to the frequency of thesignal from crystal standard 17.

In operation, the phase detector .13 receives the si-g nal from theoutput of signal source 9 and the resultant signal appearing at theoutput of \mixer 19 and produces an error signal at its output 24 whichis related to the phase difference between the output freqency f and aharmonic, say the M harmonic of the resultant signal from mixer 19. Thiserror signal is applied to .a frequency-determining element of thesignal source 9 to maintain its output frequency at the proper value.Zero average frequency error is obtained with respect to the resultantor reference signal appearing at the output of mixer 19 when the outputfrequency is stabilized in this manner. Coarse tuning of the signalsource 9 using the control 11 results in stepped variations in theoutput frequency over a wide tuning range, as shown in graph 18. This isthe result of phase lock being established between the output frequencyapplied to one input of phase detector 13 and successive harmonics ofthe reference signal appearing at the output of mixer 19 as the tuningcontrol 11 is varied. The frequency amplitude of the steps is thedifference between successive harmonic frequencies of this referencesignal. Fine frequency variations may be obtained by varying thefrequency of the variable frequency oscillator 21 using the fine tuningcontrol 22 as shown in graph 31). This causes a variation in thefrequency of the reference signal appearing at the output of mixer 19.Adjustment of the fine tuning control 22 has the effect of shifting theentire plot of output frequency versus coarse tuning up or down on thefrequency scale (broken-line curves on graph 18) by an amount at leastgreater than the height of a single step. The output frequency is thuscontinuously phase locked to the reference signal at the output of mixer19 as the output frequency is varied over a wide range of frequenciesusing both the coarse tuning and fine tuning adjustments. This referencesignal has frequency stability which approaches that of the crystalstandard 17 where the variable frequency portion (6) of the referencefrequency is very small compared with the frequency of crystal standard17.

The present invention also provides highly accurate direct indicationsof the output frequency on the counters 29 and 33 using low frequencycounting techniques. The conventional scheme of mixing a high frequencysignal with a local oscillator signal to produce a beat frequency signalis inaccurate because of the lack of information regarding the harmonicof the local oscillator against which the high frequency signal isbeating. This harmonic number information is retained in the presentinvention by combining in mixer 25 the frequency of the crystal standard417 with the signal from frequency divider 28 which has a frequencyrelated by a factor K to the freqency of the oscillator 21. Theresultant signal is combined in phase detector 15 with the outputfrequency to produce a beat frequency which contains the harmonic number(M) information and the information regarding the frequency variations(6). The harmonic number may be obtained directly as the ratio of thefrequency of the output from phase detector 15 and the frequency of theoutput of oscillator 21. In practice, this ratio is obtained digitallyby counting in counter 29 the signal at the output of phase detector 15for a gate period which is determined by the frequency of oscillator 21.This gate signal produced by divider and gate generator 35 includes ascale factor which enables the counter 29 to indicate directly theharmonic multiples (M) of the frequency i Since frequency f is aconstant in the system, it may be included in the scale factor providedby gate generator 35. The signal at the output of phase detector 15 isalso counted in counter 33 for a gate period which is determined by thefrequency of the crystal standard 17. The divider .and gate generator 37includes a scale factor which permits the counter 33 to indicate theportion of the output frequency M (f +6) which is provided by oscillator21. Where the frequency variation covered by the oscillator 21 isdecimally related to the frequency of crystal standard 17, say by afactor of one one hundredth, the two readings provided by counters 29and 33, each with the proper scale factor, may be combined to provide adirect indication of the output frequency with six place accuracy. Also,where greater frequency stability in the output signal appearing atterminal 20 is required, the oscillator 21 may be stabilized by an errorsignal derived from error network 39. In one embodiment, this networkproduces an error signal from the comparison of a voltage related to theleast significantly digits of counter 33 and a reference voltage fromsupply 41. In another embodiment, the output frequency from detector 15is compared in an error network 39 with a reference frequency fromsupply 41.

The circuit of the present invention may also be used in an alternatemode to indicate the unknown frequency of an applied signal. In thismode, signal source 9 is removed, the unknown frequency is applied atthe terminal 20, and switch 26 is set in the alternate position fromthat shown. Phase lock between the applied unknown signal and the outputof the variable frequency oscillator 21 is established and the circuitprovides a direct indication of the unknown frequency in a mannersimilar to that previously described for operation of the circuit withsignal source 9 in place.

Therefore the circuit of the present invention provides an output signalhaving a frequency which is variable over a wide band of frequencies andwhich has the degree of frequency stability comparable to the referencefrequency to which the high frequency signal is phase locked. Also, thepresent circuit provides a direct indication of the high frequencysignal using low frequency counter techniques.

We claim: 1. Signal apparatus comprising: an input circuit for receivinga signal frequency from a source;

a source of reference frequency;

means connected to the source for receiving said reference frequency andto the input circuit for receiving the signal frequency at said inputcircuit for producing a control signal representative of the phaserelationship between the signal frequency at said input circuit and aninteger multiple of the reference frequency;

means connected to one of said sources for applying said control signalthereto to maintain phase lock between the signal frequency at the inputcircuit and an integer multiple of the reference frequency;

circuit means having an input connected to the input circuit forreceiving the signal frequency and having another input;

a source of first frequency equal to the reference frequency offset by aselected frequency value;

means connecting the source of first frequency and the other input ofsaid circuit means for applying thereto a frequency proportional to thefirst frequency, said circuit means producing an output signal as amodulation product of the frequencies of signals applied to the inputsthereof;

a source of second signal proportional to said selected frequency value;and

means connected to said circuit means and to said source of secondsignal for producing a first output as the combination of said outputsignal and second signal, the first output being representative of thenumber of the harmonic of the reference frequency to which the signalfrequency at the input circuit is phase locked.

2. Signal apparatus as in claim 1 wherein:

the signal frequency is received at the input circuit from a source tobe stabilized; and

said control signal is applied to the source of signal frequency to bestabilized to alter the frequency thereof to maintain phase lock betweensaid signal frequency and said reference frequency; and comprising:

a reference signal supply; and

means connected to said supply to receive the reference signal therefromand to said circuit means to receive the output signal therefrom forapplying to said source of reference frequency an error signal producedas the combination of the reference signal and output signal to correctfor variations in the reference frequency about a value proportional tothe reference signal from said supply.

3. Signal apparatus as in claim 1 comprising:

a source of standard frequency;

a generator for producing a signal of selectable frequency;

a pair of frequency dividers connected to said generator, one of thefrequency dividers dividing the signal of selectable frequency from thegenerator by a factor K and the other of the frequency dividers dividingthe signal of selectable frequency from said generator by a factor Kin,where n is an integer not less than 1;

a pair of mixers, each connected to receive the standard frequency fromsaid source and the signal at the output of a freqency divider, one ofthe mixers producing said reference frequency at the output thereof andthe other of said mixers producing the first frequency at the outputthereof; and

said control signal is applied to said generator for altering theselectable frequency of the signal therefrom to maintain phase lockbetween the signal frequency at the input circuit and an integermultiple of the reference frequency.

4. Signal apparatus as in claim 1 comprising:

a source of standard frequency signal;

means connected to said circuit means .and to said source of standardfrequency signal for producing a second output as the combination ofsaid output signal and standard frequency signal, the second outputbeing representative of the frequency separation of the signal frequencyat the input circuit from said standard frequency.

5. Signal apparatus as in claim 4 comprising:

a source of variable frequency;

means including a first mixer connected to the sources of standard andvariable frequencies for producing said reference frequency as amodulation product of the frequencies applied to the first mixer; and

a second mixer having an input connected to said source of standardfrequency and having another input;

means connected to said source of variable frequency and to the otherinput of said second mixer for applying thereto a frequency which isrelated to said variable frequency by a constant, non-unityproportionality factor, said second mixer producing said first frequencyas a modulation product of the frequencies of signals applied to theinputs thereof;

said second output being representative of the frequency differencebetween the signal frequency at the input circuit and a selectedharmonic of the standard frequency.

6. Signal apparatus as in claim 5 comprising:

a generator for producing a signal of selectable frequency;

a pair of frequency converters connected to said generator for producingat the outputs thereof said variable frequency and said frequency whichis related to said variable frequency by a constant, nonunityproportionality factor;

one of said frequency converters converting the frequency of said signalof selectable frequency by a factor K; and

the other of said frequency converters converting the frequency of saidsignal of selectable frequency by a factor Kin Where n is an integer notless than 1.

7. Signal apparatus as in claim 5 wherein:

said means for producing the first and second outputs each includes adigital logic circuit for producing a digital output proportional to thenumber of signals applied thereto; and comprising:

means connected to said circuit means and to the digital logic circuitproviding the first output to apply thereto a frequency proportional tosaid output signal for a period proportional to said selected frequencyvalue; and

means connected to said circuit means and to the digital logic circuitproviding the second output to apply thereto a frequency proportional tosaid output signal for a period proportional to said standard frequency.

References Cited by the Examiner UNITED STATES PATENTS 2,957,144 10/1960Huhn 331-40 2,964,714 12/1960 Jakubowics 3312 2,982,921 5/ 1961 Rozneret a1. 33 1-17 ROY LAKE, Primary Examiner.

4,5 I. KOMINSKI, Assistant Examiner,

1. SIGNAL APPARATUS COMPRISING; AN INPUT CIRCUIT FOR RECEIVING A SIGNALFREQUENCY FROM A SOURCE; A SOURCE OF REFERENCE FREQUENCY; MEANSCONNECTED TO THE SOURCE FOR RECEIVING SAID REFERENCE FREQUENCY AND TOTHE INPUT CIRCUIT FOR RECEIVING THE SIGNAL FREQUENCY AT SAID INPUTCIRCUIT FOR PRODUCING A CONTROL SIGNAL REPRESENTATIVE OF THE PHASERELATIONSHIP BETWEEN THE SIGNAL FREQUENCY AT SAID INPUT CIRCUIT AND ANINTEGER MULTIPLE OF THE REFERENCE FREQUENCY; MEANS CONNECTED TO ONE OFSAID SOURCES FOR APPLYING SAID CONTROL SIGNAL THERETO TO MAINTAIN PHASELOCK BETWEEN THE SIGNAL FREQUENCY AT THE INPUT CIRCUIT AND AN INTEGERMULTIPLE OF THE REFERENCE FREQUENCY; CIRCUIT MEANS HAVING AN INPUTCONNECTED TO THE INPUT CIRCUIT FOR RECEIVING THE SIGNAL FREQUENCY ANDHAVING ANOTHER INPUT; A SOURCE OF FIRST FREQUENCY EQUAL TO THE REFERENCEFREQUENCY OFFSET BY A SELECTED FREQUENCY VALUE; MEANS CONNECTING THESOURCE OF FIRST FREQUENCY AND THE OTHER INPUT OF SAID CIRCUIT MEANS FORAPPLYING THERETO A FREQUENCY PROPORTIONAL TO THE FIRST FREQUENCY, SAIDCIRCUIT MEANS PRODUCING AN OUTPUT SIGNAL AS A MODULATION PRODUCT OF THEFREQUENCIES OF SIGNALS APPLIED TO THE INPUTS THEREOF; A SOURCE OF SECONDSIGNAL PROPORTIONAL TO SAID SELECTED FREQUENCY VALUE; AND MEANSCONNECTED TO SAID CIRCUIT MEANS AND TO SAID SOURCE OF SECOND SIGNAL FORPRODUCING A FIRST OUTPUT AS THE COMBINATION OF SAID OUTPUT SIGNAL ANDSECOND SIGNAL, THE FIRST OUTPUT BEING REPRESENTATIVE OF THE NUMBER OFTHE HARMONIC OF THE REFERENCE FREQUENCY TO WHICH THE SIGNAL FREQUENCY ATTHE INPUT CIRCUIT IF PHASE LOCKED.